Well yes, my point is that you can draw the line in any number of places. Even if we were to agree upon your definition — and there are many different, conflicting definitions of 'mental illness' — what constitutes a significant impact on quality of life?

At some point you have to draw an arbitrary line, and even then it's frequently a guessing game working out on which side of the line someone actually lies.

My definition is from the DSM and while it is true that there's always edge cases, in practice the gray area is pretty small. Especially when we're talking about serious things like schizophrenia.